Heat oven to 350°F. Prepare two 8x8-inch square baking pans with butter & flour or GOOP or non-stick spray.
In a medium bowl, sift together flour, cocoa, baking powder, baking soda, and salt. Set Aside.
In a small bowl combine buttermilk and pumpkin. Whisk together until incorporated.
In a mixer, beat the butter, sugars, and vanilla together until light and fluffy. Add in eggs, one at a time, until fully incorporated.
Alternatively add the flour mixture and buttermilk mixture to the sugar mixture, beginning and ending with the flour mixture.
Mix for 1 minute on low until fully combined. Divide batter evenly between two prepared baking pans.
Bake for 18-26 minutes. The cake is done when an inserted toothpick comes out mostly clean.